* Extremely low oil level, not completely empty: There might have been a tiny amount of oil remaining in the sump, enough to provide minimal lubrication for a short period. This is dangerous and should never be relied upon.
* Oil pressure sensor malfunction: The oil pressure sensor might be faulty, giving a false reading. The engine could still be severely damaged despite starting.
* Incorrect oil level indication: The dipstick could be inaccurate, or you might have misread it.
* Severe damage not immediately apparent: The damage could be so extensive that it's not preventing the engine from turning over initially. This doesn't mean the damage isn't present; it'll likely manifest as serious problems very soon.
Regardless of the reason, running a diesel engine low on oil is incredibly damaging. The engine is likely to have sustained considerable internal damage, even if it started. Continuing to run it risks complete seizure and massive repair costs or even replacement.
Do not run the engine again. Have a qualified mechanic inspect it immediately to assess the damage. Expect significant repairs to be needed. Continuing to drive it is extremely risky and will likely result in much higher repair bills later.
